Abstract:

An exemplary hydraulic system includes a plurality of digital valves, each
valve fluidly connectable to a corresponding hydraulic load. The digitals
valves are operable to fluidly connect the corresponding hydraulic load
to a pressure source. The hydraulic system further includes a digital
controller operably connected to the plurality of digital valves. The
digital controller is configured to assign a priority level so that it is
associated with each of a plurality of hydraulic loads, and to formulate
a pulse width modulated control signal based on the assigned priority
levels. The digital controller transmits the control signal to the
plurality of digital valves for controlling the operation of the valves.

Claims:

1. A method comprising:assigning a priority level so that it is associated
with each of a plurality of hydraulic loads;formulating a pulse width
modulated control signal based on the assigned priority
levels;transmitting the control signal to a plurality of digital valves,
each valve operable to selectively fluidly connect at least one of the
hydraulic loads to a pressure source; andsequentially actuating at least
a subset of the digital valves in response to the control signal.

2. The method of claim 1 further comprising actuating each of the digital
valves no more than once over a single operating cycle.

3. The method of claim 1, wherein the control signal defines periods of
time during an operating cycle in which the respective valves are
arranged in an open position and a closed position.

4. The method of claim 3, where each valve is opened and closed no more
than once during each operating cycle.

5. The method of claim 2, further comprising actuating the valves in
sequential order based on the assigned priority level of the associated
hydraulic load.

6. The method of claim 5, wherein the valve associated with the hydraulic
load having a highest priority level is actuated first.

7. The method of claim 5, further comprising basing each assigned priority
level on a pressure requirement of the specific hydraulic load.

8. The method of claim 7, wherein the valve associated with the hydraulic
load having the highest pressure requirement is actuated first.

9. The method of claim 7, further comprising sequentially actuating the
valves commencing with the valve associated with the hydraulic load
having the highest pressure requirement and proceeding in sequential
descending order based on the pressure requirements of the remaining
hydraulic loads.

10. The method of claim 7, further comprising sequentially actuating the
valves commencing with the valve associated with the hydraulic load
having the lowest pressure requirement and proceeding in sequential
ascending order based on the pressure requirements of the remaining
hydraulic loads.

11. The method of claim 1, wherein the formulating of the control signal
includes determining a duty cycle for each of the digital valves defining
time periods during which the valves are arranged in a closed position
and an open position.

12. The method of claim 11, further comprising:determining a flow
requirement for each of the plurality of hydraulic loads; anddetermining
a duty cycle for each of the valves calculated to produce the flow
requirement of the associated hydraulic load.

13. The method of claim 12, wherein at least one of the control valves is
assigned a duty cycle determined to produce less than the flow
requirement of the associated hydraulic load when the total flow
requirement of all the hydraulic loads is greater than an available flow
of pressurized fluid.

14. The method of claim 11, wherein the duty cycle is determined based on
the flow requirement of the associated hydraulic load.

15. The method of claim 11, wherein the duty cycle for each of the digital
valves is determined prior to commencing an operating cycle.

16. The method of claim 15, wherein the duty cycle for each of the digital
valves is maintained throughout the operating cycle.

17. The method of claim 15 further comprising:evaluating the duty cycle
for each valve prior to actuating the respective valve; andmodifying the
duty cycle determined prior to commencing the operating cycle based on
the flow requirement of the associated hydraulic load.

18. A hydraulic system comprising:a plurality of digital valves, each
valve fluidly connectable to a corresponding hydraulic load, the digitals
valves operable to fluidly connect the corresponding hydraulic load to a
pressure source; anda digital controller operably connected to the
plurality of digital valves, the digital controller configured to assign
a priority level so that it is associated with each of a plurality of
hydraulic loads and formulate a pulse width modulated control signal
based on the assigned priority levels, the digital controller operable to
transmit the control signal to the plurality of digital valves for
controlling the operation of the valves.

19. The hydraulic system of claim 18, wherein the control signal is
formulated to actuate each of the digital valves no more than once over a
single operating cycle.

20. The hydraulic system of claim 19, wherein the control signal defines
periods of time during each operating cycle in which the respective
valves are arranged in an open position and a closed position.

21. The hydraulic system of claim 20, where each valve is opened and
closed no more than once during each operating cycle.

22. The hydraulic system of claim 19, wherein the controller is configured
to actuate the valves in sequential order based on the assigned priority
level of the associated hydraulic load.

23. The hydraulic system of claim 22, wherein the valve associated with
the hydraulic load having a highest priority level is actuated first.

24. The hydraulic system of claim 22, wherein the controller is configured
to assign the priority levels based on a pressure requirement of the
hydraulic loads.

25. The hydraulic system of claim 24, wherein the valve associated with
the hydraulic load having the highest pressure requirement is actuated
first.

26. The hydraulic system of claim 24, wherein the controller is configured
to sequentially actuate the valves commencing with the valve associated
with the hydraulic load having the highest pressure requirement and
proceeding in sequential descending order based on the pressure
requirements of the remaining hydraulic loads.

27. The hydraulic system of claim 24, wherein the controller is configured
to sequentially actuate the valves commencing with the valve associated
with the hydraulic load having the lowest pressure requirement and
proceeding in sequential ascending order based on the pressure
requirements of the remaining hydraulic loads.

28. The hydraulic system of claim 18, wherein the controller is configured
to determine a duty cycle for each of the digital valves defining time
periods during which the valves are arranged in a closed position and an
open position.

29. The hydraulic system of claim 28, wherein the controller is configured
to determine a flow requirement for each of the plurality of hydraulic
loads and determine a duty cycle for each of the valves calculated to
produce the flow requirement of the associated hydraulic load.

30. The hydraulic system of claim 29, wherein at least one of the control
valves is assigned a duty cycle determined to produce less than the flow
requirement of the associated hydraulic load when the total flow
requirement of all the hydraulic loads is greater than an available flow
of pressurized fluid.

31. The hydraulic system of claim 28, wherein the duty cycle is determined
based on the flow requirement of the associated hydraulic load.

32. The hydraulic system of claim 28, wherein the duty cycle for each of
the digital valves is determined prior to commencing an operating cycle.

33. The hydraulic system of claim 32, wherein the duty cycle for each of
the digital valves is maintained throughout the operating cycle.

34. The hydraulic system of claim 32, wherein the controller is configured
to evaluate the duty cycle for each valve prior to actuating the
respective valve, and modify the duty cycle determined prior to
commencing the operating cycle based on the flow requirement of the
associated hydraulic load.

[0002]A hydraulic system may include multiple hydraulic loads, each of
which may have different flow and pressure requirements that can vary
over time. The hydraulic system may include a pump for supplying a flow
of pressurized fluid to the hydraulic loads. The pump may have a variable
or fixed displacement configuration. Fixed displacement pumps are
generally smaller, lighter, and less expensive than variable displacement
pumps. Generally speaking, fixed displacement pumps deliver a definite
volume of fluid for each cycle of pump operation. But depending on the
configuration of the pump and the precision with which the pump is
manufactured, the flow output of the pump may actually decrease as the
system pressure level increases due to internal leakage from the outlet
side to the inlet side of the pump. The output volume of a fixed
displacement pump can be controlled by adjusting the speed of the pump.
Closing or otherwise restricting the outlet of a fixed displacement pump
will cause a corresponding increase in the system pressure. To avoid over
pressurizing the hydraulic system, fixed displacement pumps typically
utilize a pressure regulator or an unloading valve to control the
pressure level within the system during periods in which the pump output
exceeds the flow requirements of the multiple hydraulic loads. The
hydraulic system may further include various valves for controlling the
distribution of the pressurized fluid to the multiple loads.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]FIG. 1 is a schematic representation of an exemplary hydraulic
system including a fixed displacement pump for driving multiple hydraulic
loads.

[0004]FIG. 2 is a graphical depiction of exemplary duty cycles employed by
multiple control valves for controlling the distribution of pressurized
fluid to the multiple hydraulic loads.

[0005]FIG. 4 is a graphical depiction of relative pump output pressure
levels that may occur when employing the exemplary valve duty cycles
illustrated in FIG. 2.

[0006]FIG. 5 is a graphical depiction of an exemplary sequencing of the
control valves employed with the hydraulic system.

[0007]FIGS. 6A and 6B are graphical depictions of changes to the valve
sequencing order shown in FIG. 5 to accommodate changes in the pressure
requirements of the hydraulic loads.

[0008]FIGS. 7A and 7B are graphical depictions of the effect of time delay
on system pressure.

[0009]FIGS. 8A and 8B are graphical depictions of an exemplary
implementation of progressive pulse width control.

[0010]FIG. 9 is a graphical depiction of an exemplary pressure drop
occurring across three separate controls valves operated in succession.

[0011]FIG. 10 graphically depicts a Time Delay Pressure Error computed
based on the corresponding pressure drops presented in FIG. 9.

[0012]FIG. 11 is an enlarged view of a portion of FIG. 9 depicting the
transition period between the closing of one control valve and the
opening of the next subsequent control valve.

DETAILED DESCRIPTION

[0013]Referring now to the discussion that follows and also to the
drawings, illustrative approaches to the disclosed systems and methods
are shown in detail. Although the drawings represent some possible
approaches, the drawings are not necessarily to scale and certain
features may be exaggerated, removed, or partially sectioned to better
illustrate and explain the present invention. Further, the descriptions
set forth herein are not intended to be exhaustive or otherwise limit or
restrict the claims to the precise forms and configurations shown in the
drawings and disclosed in the following detailed description.

[0014]FIG. 1 schematically illustrates an exemplary hydraulic system 10
for controlling multiple fluid circuits incorporating multiple hydraulic
loads having variable flow and pressure requirements. Pressurized fluid
for driving the hydraulic loads is provided by a hydraulic fixed
displacement pump 12. Pump 12 may include any of a variety of known fixed
displacement pumps, including but not limited to, gear pumps, vane pumps,
axial piston pumps, and radial piston pumps. Pump 12 includes a drive
shaft 14 for driving the pump. Drive shaft 14 can be connected to an
external power source, such as an engine, electric motor, or another
power source capable of outputting a rotational torque. An inlet port 16
of pump 12 is fluidly connected to a fluid reservoir 18 through a pump
inlet passage 20. A pump discharge passage 22 is fluidly connected to a
pump discharge port 24. Although a single pump 12 is illustrated for
purposes of exemplary illustration, hydraulic system 10 may include
multiple pumps, each having their respective discharge ports fluidly
connected to a common fluid node from which the individual fluid circuits
can be supplied with pressurized fluid. The multiple pumps may be fluidly
connected, for example, in parallel to achieve higher flow rates, or in
series, such as when higher pressures for a given flow rate are desired.

[0015]Pump 12 is capable of generating a flow of pressurized fluid that
can be used to selectively drive multiple hydraulic loads. For purposes
of illustration, hydraulic system 10 is shown to include three separate
hydraulic loads, although it shall be appreciated that fewer or more
hydraulic loads may also be provided depending on the requirements of the
particular application. By way of example, the three hydraulic loads may
include a hydraulic cylinder 26, a hydraulic motor 28, and a
miscellaneous hydraulic load 30, which may include any of a variety of
hydraulically actuated devices. Of course, it shall be appreciated that
other types of hydraulic loads may also be used in place of, or in
combination with, one or more of the illustrative hydraulic loads 26, 28
and 30, depending on the requirements of the particular application.

[0016]Each hydraulic load 26, 28, and 30 may be associated with a separate
fluid circuit. A first fluid circuit 32 includes hydraulic cylinder 26; a
second fluid circuit 34 includes hydraulic motor 28; and a third fluid
circuit 36 includes miscellaneous hydraulic load 30. In the exemplary
illustration the three fluid circuits are fluidly connected in parallel
to pump discharge passage 22 at fluid junction 38.

[0017]Each fluid circuit includes a control valve, illustrated as a
digital control valve, for individually controlling the operation of the
hydraulic load associated with the respective fluid circuit. The control
valve may control a time averaged flow rate passing through each of the
respective fluid circuits and the corresponding pressure levels. Each
control valve may include an actuator, which when activated opens the
respective control valve to allow pressurized fluid to pass through the
control valve to the associated hydraulic load. When utilizing a time
averaged flow rate approach, the rate at which fluid passes through the
control valve is controlled by repetitively cycling the control valve
(i.e., opening and closing the valve) using a method commonly known as
pulse width modulation ("PWM"). The control valve is either fully open or
fully closed at any given time when employing pulse width modulation. The
time averaged flow rate through the control valve, and corresponding
pressure levels, may be controlled by adjusting the time periods during
which the control valve is open and closed, also known as the valve duty
cycle. For example, a duty cycle in which the valve is open generally
fifty (50) percent of the time will generally produce a time averaged
flow rate of approximately fifty (50) percent of the control pump's
instantaneous flow output. Inherent fluctuations in the control valve's
flow output tend to decrease as the operating frequency of the control
valve increases. The inherent fluctuations in the control valve's flow
may cause a pressure ripple that may be distributed to the load. The
accumulator is generally sized such that the pressure ripples are
acceptably small for a given application. Increasing the accumulator size
may adversely affect the time required to respond to changes in load
pressure. The operating frequency of the duty cycle may be increased,
which may reduce the required accumulator size while improving both the
response time and the magnitude of the pressure ripple. If the frequency
is increased high enough, it may be possible to eliminate the accumulator
by taking advantage of the natural compliance of the oil and conveyance
to meet the pressure ripple requirement for the load. Valve operating
speed limits and increased valve power losses that reduce efficiency may
limit the operating frequency of the duty cycle.

[0018]Continuing to refer to FIG. 1, hydraulic system 10 includes a first
control valve 40 for controlling the distribution of pressurized fluid
from pump 12 to first fluid circuit 32, and in particular, to hydraulic
cylinder 26. Control valve 40 may be a digital valve that can be operated
in the manner described previously using pulse width modulation. Although
illustrated schematically in FIG. 1 as a two-way, two-position valve, it
shall be appreciated that other valve configurations may also be used
depending on the particular application. Control valve 40 includes an
inlet port 46 fluidly connected to pump discharge passage 22 at fluid
junction 38 through an inlet passage 48. Fluidly connected to a discharge
port 50 of control valve 40 is a discharge passage 52. First control
valve 40 may also include an actuator 42 operable for selectively opening
and closing a fluid path between inlet port 46 and discharge port 50 in
response to a control signal. Actuator 42 may be configured to open
control valve 40, but not close it, in which case a second actuator 43
may be employed to selectively close the valve. Actuators 42 and 43 may
have any of a variety of configurations, including but not limited to, a
pilot valve, a solenoid, and a biasing member, such as a spring.

[0019]The distribution of pressurized fluid to hydraulic cylinder 26 from
control valve 40 may be further controlled by a hydraulic cylinder
control valve 54, which is fluidly connected to control valve 40 through
discharge passage 52. Hydraulic cylinder control valve 54 operates to
selectively distribute the pressurized fluid received from control valve
40 between a first chamber 58 and a second chamber 60 of hydraulic
cylinder 26. A first supply passage 62 fluidly connects first chamber 58
to hydraulic cylinder control valve 54, and a second supply passage 64
fluidly connects second chamber 60 to hydraulic cylinder control valve
54. A reservoir return passage 66, which is fluidly connected to
hydraulic cylinder control valve 54, is provided for returning fluid
discharged from hydraulic cylinder 26 to fluid reservoir 18.

[0020]A digital valve controlled using pulse width modulation generally
does not produce a continuous flow output, but rather produces a cyclic
output in which a volume of fluid is discharged from the valve followed
by a period in which no fluid discharge is produced. To help compensate
for the cyclic output of the control valve and deliver a more uniform
flow of pressurized fluid to the hydraulic load, an accumulator 68 may be
provided. Accumulator 68 stores pressurized fluid discharged from control
valve 40 during the discharge stage of the valve duty cycle. The stored
pressurized fluid can be released during the period in which control
valve 40 is closed to compensate for the cyclic discharge of control
valve 40 and deliver a more constant flow of pressurized fluid to
hydraulic load 26.

[0021]Accumulator 68 may have any of a variety of configurations. For
example, one version of accumulator 68 may include a fluid reservoir 69
for receiving and storing pressurized fluid. Reservoir 69 can be fluidly
connected to discharge passage 52 at a fluid junction 71 through a
supply/discharge passage 73. Accumulator 68 may include a moveable
diaphragm 75. The location of diaphragm 75 within accumulator 68 can be
adjusted to selectively vary the volume of reservoir 69. A biasing
mechanism 79 urges diaphragm 75 in a direction that tends to minimize the
volume of reservoir 69 (i.e., away from biasing mechanism 79). Biasing
mechanism 79 exerts a biasing force that opposes the pressure force
exerted by the pressurized fluid present within reservoir 69. If the two
opposing forces are unbalanced, diaphragm 75 will be displaced to either
increase or decrease the volume of reservoir 69, thereby restoring
balance between the two opposing forces. For example, when control valve
40 is opened the pressure level at fluid junction 71 will tend to
increase. Generally speaking, the pressure level within reservoir 69
corresponds to the pressure at fluid junction 71. If the pressure force
within reservoir 69 exceeds the opposing force generated by biasing
mechanism 79, diaphragm 75 will be displaced toward biasing mechanism 79,
thereby increasing the volume of the reservoir and the amount of fluid
that can be stored in reservoir 69. As reservoir 69 continues to fill
with fluid, the opposing force generated by biasing mechanism 79 will
also increase to the point at which the biasing force and the opposing
pressure force exerted from within reservoir 69 are substantially equal.
The volumetric capacity of reservoir 69 will remain substantially
constant when the two opposing forces are at equilibrium. On the other
hand, closing control valve 40 will generally cause the pressure level at
fluid junction 71 to drop below the pressure level within reservoir 69.
This coupled with the fact that the pressure forces across diaphragm 75
are now unbalanced will cause fluid stored in reservoir 69 to be
discharged through supply/discharge passage 73 to discharge passage 52
and delivered to hydraulic load 26.

[0022]Hydraulic system 10 may also include a second control valve 70 for
controlling the distribution of pressurized fluid from pump 12 to second
fluid circuit 34, and in particular, to hydraulic motor 28. Control valve
70 may also be a high frequency digital valve that can be operated in the
manner described previously using pulse width modulation. Although
illustrated schematically in FIG. 1 as a two-way, two-position valve, it
shall be appreciated that other valve configurations may also be used,
depending on the requirement of the particular application. Control valve
70 includes an inlet port 72 fluidly connected to pump discharge passage
22 at a fluid junction 74 through a control valve inlet passage 76.
Control valve 70 may also include an actuator 77 operable for selectively
opening and closing a fluid path between inlet port 72 and a discharge
port 78 in response to a control signal. Actuator 77 may be configured to
open control valve 70, but not close it, in which case a second actuator
81 may be employed to selectively close the valve. Actuators 77 and 81
may have any of a variety of configurations, including but not limited
to, a pilot valve, a solenoid, and a biasing member, such as a spring.

[0023]Fluidly connected to discharge port 78 of control valve 70 is a
hydraulic motor supply passage 80 in fluid communication with hydraulic
motor 28. In turn hydraulic fluid may be discharged from hydraulic motor
28 through a discharge passage 82 fluidly connected to reservoir return
passage 66 at fluid junction 83. A second accumulator 84 may be provided
within supply passage 80 to store pressurized fluid in much the same
manner as previously described with respect to accumulator 68.
Accumulator 84 can be fluidly connected to hydraulic motor supply passage
80 at a fluid junction 85 through a supply/discharge passage 87.
Pressurized fluid discharged from control valve 70 can be used to charge
accumulator 84 during the discharge stage of control valve 70. The stored
pressurized fluid can be released during the period in which control
valve 70 is closed to help minimize fluctuations in the flow of
pressurized fluid being delivered to hydraulic load 28.

[0024]Hydraulic system 10 may also include a third control valve 86 for
controlling the distribution of pressurized fluid from pump 12 to third
fluid circuit 36. Similar to control valves 40 and 70, control valve 86
may also be a high frequency digital valve that can be operated in the
manner described previously using pulse width modulation. Although
illustrated schematically in FIG. 1 as a two-way, two-position valve, it
shall be appreciated that other valve configurations may also be used,
depending on the requirements of the particular application. An inlet
port 88 of control valve 86 is fluidly connected to pump discharge
passage 22 at a fluid junction 90 through a control valve inlet passage
92. Control valve 86 may also include an actuator 93 operable for
selectively opening and closing a fluid path between inlet port 88 and a
discharge port 96 in response to a control signal. Actuator 93 may be
configured to open control valve 86, but not close it, in which case a
second actuator 91 may be employed to selectively close the valve.
Actuators 91 and 93 may have any of a variety of configurations,
including but not limited to, a pilot valve, a solenoid, and a biasing
member, such as a spring.

[0025]A hydraulic load supply passage 94 fluidly connects discharge port
96 of control valve 86 to hydraulic load 30. Pressurized hydraulic fluid
may be discharged from hydraulic load 30 through a discharge passage 98
fluidly connected to reservoir return passage 66 at fluid junction 103.
An accumulator 95 may be provided to store pressurized fluid in much the
same manner as previously described with respect to accumulator 68.
Accumulator 95 may be fluidly connected to hydraulic load supply passage
94 at a fluid junction 97 through a supply/discharge passage 99.
Pressurized fluid discharged from control valve 86 may be used to charge
accumulator 95 during the discharge stage of control valve 86. The stored
pressurized fluid may be released when control valve 86 is closed to help
offset fluctuations in the flow of pressurized fluid to hydraulic load
30.

[0026]Closing or otherwise restricting the outlet of fixed displacement
pump 12 can cause the pressure within hydraulic system 10 to reach
undesirable levels. To avoid over pressurizing the hydraulic system
during periods in which the pump output exceeds the flow requirements of
the hydraulic loads, a bypass control valve 100 associated with a bypass
fluid circuit 101 may be provided. An inlet port 102 of bypass control
valve 100 may be fluidly connected to pump discharge passage 22 at a
fluid junction 104 through an inlet passage 106. Bypass control valve 100
is operable to selectively allow excess flow generated by pump 12 to be
dumped to fluid reservoir 18. A bypass discharge passage 108 is fluidly
connected to a discharge port 110 of bypass control valve 100 and
reservoir return passage 66 at fluid junction 111. Bypass control valve
100 also includes an actuator 112 operable for selectively opening and
closing a fluid path between inlet port 102 and discharge port 110 of
bypass valve 100 in response to a control signal. Actuator 112 may be
configured to open bypass control valve 100, but not close it, in which
case a second actuator 113 may be employed to selectively close the
valve. Actuators 112 and 113 may have any of a variety of configurations,
including but not limited to, a pilot valve, a solenoid, and a biasing
member, such as a spring.

[0027]A controller 114 may be provided for controlling the operation of
control valves 40, 70, 86 and 100. More generally, controller 114 may
form a portion of a more general system based Electronic Control Unit
(ECU) or may be in operational communication with such an ECU. Controller
114 may include, for example, a microprocessor, a central processing unit
(CPU), and a digital controller, among others.

[0028]More specifically controller 114 and any associated ECU is an
example of a device generally capable of executing instructions stored on
a computer-readable medium, such as instructions for performing one or
more of the processes discussed herein. Computer-executable instructions
may be compiled or interpreted from computer programs created using a
variety of known programming languages and/or technologies, including,
without limitation, and either alone or in combination, Java, C, C++,
Visual Basic, Java Script, Perl, etc. In general, a processor (e.g., a
microprocessor) receives instructions, e.g., from a memory, a
computer-readable medium, etc., and executes these instructions, thereby
performing one or more processes, including one or more of the processes
described herein. Such instructions and other data may be stored and
transmitted using a variety of known computer-readable media.

[0029]A computer-readable medium (also referred to as a processor-readable
medium) includes any tangible medium that participates in providing data
(e.g., instructions) that may be read by a computer (e.g., by a processor
of a computer, a microcontroller, etc.). Such a medium may take many
forms, including, but not limited to, non-volatile media and volatile
medial. Non-volatile media may include, for example, optical or magnetic
disks, read-only memory (ROM), and other persistent memory. Volatile
media may include, for example, dynamic random access memory (DRAM),
which typically constitutes a main memory. Common forms of
computer-readable media include, for example, a floppy disk, a flexible
disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD,
any other optical medium, punch cards, paper tape, any other tangible
medium with patterns of holes, a RAM, a PROM, an EPROM, a FLASH-EEPROM,
any other memory chip or cartridge, or any other medium from which a
computer can read.

[0030]A transmission media may facilitate the processing of instructions
by carrying instructions from one component or device to another. For
example, a transmission media may facilitate electronic communication
between mobile device 110 and telecommunications server 126. Transmission
media may include, for example, coaxial cables, copper wire and fiber
optics, including the wires that comprise a system bus coupled to a
processor of a computer. Transmission media may include or convey
acoustic waves, light waves, and electromagnetic emissions, such as those
generated during radio frequency (RF) and infrared (IR) data
communications.

[0031]A digital controller 14 is illustrated. A first control link 116
operably connects controller 114 to actuator 42 of control valve 40. A
second control link 117 operably connects controller 114 to actuator 43
of control valve 40. A third control link 118 operably connects
controller 114 to actuator 77 of control valve 70. A fourth control link
119 operably connects controller 114 to actuator 81 of control valve 70.
A fifth control link 120 operably connects controller 114 to actuator 93
of control valve 86. A sixth control link 121 operably connects
controller 114 to actuator 91 of control valve 86. A first bypass control
link 122 operably connects controller 114 to actuator 112 of bypass
control valve 100. A second bypass control link 123 operably connects
controller 114 to actuator 113 of bypass control valve 100. Controller
114 may be configured to control operation of the control valves in
response to various system inputs, such as the pressure and flow
requirements of the hydraulic loads, pump speed, pump exit pressure, and
the discharge fluid flow rate from pump 12, among others. Depending on
the requirements of the particular application, hydraulic system 10 may
include various sensors for monitoring various operating characteristics
of the system, and may include a speed sensor 124, a pressure sensor 126,
and a flow sensor 128, as well as others.

[0032]Control valves 40, 70, 86, and 100 may be digitally controlled using
pulse width modulation. Generally, the control valves are either fully
open or fully closed when employing pulse with modulation. Also,
typically only one control valve is fully open at any given instance,
although a portion of the opening and closing sequences of consecutive
valves may occur simultaneously, which is discussed in more detail
subsequently. Substantially the entire quantity of fluid discharged from
pump 12 passes through the control valve when the valve is open.
Operating the control valve in this manner results in a generally cyclic
fluid output, in which either the entire fluid output of pump 12 is
discharged from the control valve or none at all. Control valves 40, 70,
86, and 100 are typically operated at a relatively high operating
frequency. The operating frequency is defined as the number of duty
cycles completed per unit of time, typically expressed as cycles/sec or
Hertz.

[0033]The effective flow rate of fluid passing through control valves 40,
70, 86 and 100 can be controlled by adjusting the respective valve duty
cycle. A complete duty cycle includes one opening and one closing of the
control valve. The duty cycle can be expressed as the ratio of the time
period that the control valve is open and the duty cycle operating
period. The duty cycle operating period may be defined as the time
required to complete one duty cycle. The duty cycle is typically
expressed as a percentage of the operating period. For example, a
seventy-five percent (75%) duty cycle results in the control valve being
open approximately seventy-five percent (75%) of the time and closed
twenty-five percent (25%) of the time. The term "effective flow rate"
refers to the time averaged flow rate of fluid discharged from the
control valve over one complete duty cycle expressed as a percentage of
the flow output of pump 12. The effective flow rate is determined by
dividing the total quantity of fluid discharged from the control valve
over one complete duty cycle by the duty cycle operating period. For
example, operating the control valve at a seventy-five percent (75%) duty
cycle will produce an effective discharge flow rate of seventy-five
percent (75%) of the flow output of pump 12.

[0034]Exemplary duty cycles for control valves 40, 70, 86 and 100 are
shown in FIG. 2. It shall be understood that the duty cycles shown in
FIG. 2 are representative duty cycles selected for the purpose of
discussing and illustrating various aspects of the hydraulic system. In
practice, the duty cycle for a given control valve will likely vary from
that which is illustrated, and indeed, any or all of the duty cycles may
be continuously varied to accommodate changing operating requirements of
the various hydraulic loads.

[0035]The duty cycles employed with each of the control valves 40, 70, 86,
and 100, may be reevaluated for each operating cycle and adjusted as
necessary to accommodated changing load conditions. Factors that may be
considered in determining the appropriate duty cycles for control valves
40, 70, 86 and 100 may include the flow and pressure requirements of
hydraulic loads 26, 28 and 30, the flow output of pump 12, the discharge
pressure of pump 12, and the operating speed of pump 12, as well as
others.

[0036]The duty cycle tracks a generally square waveform represented by a
solid line in FIG. 2. The duty cycles for each of the control valves
generally have the same operating period. For purposes of discussion, an
operating period of 20 milliseconds is illustrated in FIG. 2. In
practice, however, a longer or shorter operating period may be selected
depending on the configuration of hydraulic system 10 and the
requirements of the particular application in which the hydraulic system
is used, provided that each of the control valves generally employs the
same operating period. The operating period may be continuously varied to
accommodate changing operating conditions.

[0037]The effective flow rate of control valves 40, 70, 86 and 100 may be
controlled by varying their respective duty cycles. The duty cycle for
each of the control valves 40, 70, 86 and 100 may be continuously varied
to accommodate changing load conditions. Controller 114 may be configured
to determine the duty cycle for each of the control valves. Controller
114 may also be configured to transmit a control signal corresponding to
the desired duty cycle that may be used to control operation of the
respective control valve. Controller 114 may include logic for
determining an appropriate duty cycle based on a variety of inputs.

[0038]The control strategy employed by controller 114 may be based on an
open-loop or closed-loop control scheme. In a closed-loop system,
controller 114 may receive feedback information from a variety of sensors
used to monitor various operating parameters, such as pressure,
temperature, and speed, to name a few. Controller 114 may use the
information received from the sensors to adjust, if necessary, the duty
cycle of the respective control valve to achieve a desired load
performance. A closed-loop system may allow various operating parameters,
such as pressure, speed, and flow, to be controlled more precisely. A
closed loop system may be used, for example, to control the pressure
applied to hydraulic load 30. Controller 114 may receive feedback
information from a pressure sensor 138 regarding the actual pressure
applied to hydraulic load 30. A communication link 139 operably connects
pressure sensor 138 to controller 114. Controller 114 may use the
pressure data to compute a pressure error corresponding to the difference
between the pressure commanded by controller 114 and the pressure applied
to hydraulic load 30, as detected by pressure sensor 138. If the pressure
error falls outside a selected error range controller 114 can modify the
duty cycle of control valve 86 to achieve the desired pressure at
hydraulic load 30.

[0039]A closed loop system may also be used to implement a load sensing
control scheme. A hydraulic system employing load sensing has the ability
to monitor the system pressures and to make appropriate adjustments as
necessary to provide a desired flow rate at a pressure required to
operate the hydraulic load. Load sensing may be implemented by monitoring
a pressure drop across an orifice positioned within a passage supplying
pressurized fluid to the hydraulic load. The pressure drop across the
orifice is generally set at a predetermined fixed value. With the
pressure drop across the orifice fixed, the flow rate through the orifice
is only dependent on the flow area of the orifice. This enables the rate
at which fluid is delivered to the hydraulic load to be controlled by
adjusting the cross-sectional flow area of the orifice while maintaining
the desired constant pressure drop. Increasing the orifice
cross-sectional flow area increases the flow rate, whereas decreasing the
orifice cross-sectional flow area decreases the flow rate. A change in
the pressure drop across the orifice, which may be due for example, to an
increase in the working load being moved by the hydraulic load, will
cause a corresponding change in the flow rate of fluid delivered to the
hydraulic load. The change in pressure drop across the orifice may be
detected and compensated for by adjusting the upstream orifice pressure
to achieve the desired pressure drop.

[0040]Load sensing capabilities may be advantageous when trying to control
a hydraulic device requiring a particular flow while maintaining a
particular pressure drop across a metering orifice. Hydraulic cylinder 26
is an example of such a device. Hydraulic cylinder 26 may be used in a
variety of applications. By way of example and for purposes of
discussion, hydraulic cylinder 26 will be described in the context of a
power steering system, although it shall be appreciated that other
applications of hydraulic cylinder 26 may also be possible. Hydraulic
cylinder 26 may include a piston 140 slidably disposed in a cylinder
housing 141. An end 142 of piston 140 is connected through a series of
links to a wheel of the vehicle. Piston 140 may be slid longitudinally
within cylinder housing 141 by selectively delivering pressurized fluid
to first and second chambers 58 and 60. The rate at which the fluid is
delivered to the respective chambers determines the speed at which piston
140 moves. Hydraulic cylinder control valve 54 operates to distribute the
pressurized fluid between fluid chambers 58 and 60 of hydraulic cylinder
26. Hydraulic cylinder control valve 54 includes a variable orifice that
controls the rate at which fluid is delivered to hydraulic cylinder 26.
Hydraulic cylinder control valve 54 is responsive to a user input that
causes the valve to adjust the orifice size to achieve a desired flow
rate and to direct the flow to the appropriate chamber in hydraulic
cylinder 26.

[0041]A load sensing control scheme may be implemented by arranging a pair
of pressure sensors 144 and 146 upstream and downstream, respectively, of
hydraulic cylinder control valve 54. A first communication link 145 and a
second communication link 147 may operably connect pressure sensors 144
and 146, respectively, to controller 114. The pressure sensors may be
configured to send a pressure signal to controller 114 indicative of the
pressure at the respective sensor locations. Controller 114 uses the
pressure data to formulate an appropriate control signal, using logic
included in controller 114, for controlling the operation of control
valve 40. The control signal includes a pulse width modulated signal that
can be sent to actuator 42 across control link 116. Actuator 42 opens and
closes control valve 40 in response to the received signal. Controller
114 determines an appropriate pulse width for the control signal that is
calculated to deliver a desired flow at a desired pressure margin to
hydraulic cylinder control valve 54. Controller 114 monitors the pressure
drop across the orifice in hydraulic cylinder control valve 54 and may
adjust the control signal as necessary to maintain the desired pressure
drop across the orifice. For example, increasing the opposing force
applied to end 142 of piston 140 may cause a corresponding increase in
the downstream pressure monitored by pressure sensor 146 and a
corresponding decrease in the pressure drop across the orifice in
hydraulic cylinder control valve 54. The decreased pressure drop may also
result in a corresponding decrease in the flow rate of fluid to hydraulic
cylinder 26. To compensate for the decrease in flow, controller 114 may
increase the pressure at the inlet to hydraulic cylinder control valve
54, which is monitored using pressure sensor 144, by adjusting the duty
cycle of the control signal that controls the operation of control valve
40. The pressure to the inlet may be increased an amount sufficient to
achieve the same pressure drop across the orifice that was present before
the opposing force applied to end 142 of piston 140 was increased. In
this way, the desired flow rate delivered to hydraulic cylinder 26, and
thus the actuating speed of the piston, can be maintained at the desired
level notwithstanding the fact the forces acting against the piston are
continuously fluctuating.

[0042]A closed loop system may also be used to control the speed of a
hydraulic device, such as hydraulic motor 28. Controller 114 may receive
feedback information from a speed sensor 148 indicating the rotational
speed of hydraulic motor 28. A communication link 149 operably connects
speed sensor 148 to controller 114. Controller 114 may use the speed data
to compute a speed error corresponding to the difference between a speed
commanded by controller 114 and the actual rotational speed of hydraulic
motor 28, as detected by speed sensor 148. If the speed error falls
outside a selected error range, controller 114 may modify the duty cycle
of control valve 70 in order to operate hydraulic motor 28 at the desired
speed.

[0043]A closed loop system may also be used to control the flow rate of
hydraulic fluid delivered to a hydraulic device, such as hydraulic device
30. Controller 114 may receive feedback information from a flow sensor
150 indicating the flow rate of fluid delivered to hydraulic device 30. A
communication link 151 operably connects flow sensor 150 to controller
114. Controller 114 may use the flow data to compute a flow error
corresponding to the difference between a flow rate commanded by
controller 114 and an actual flow rate as detected by flow sensor 150. If
the flow error falls outside a selected error range, controller 114 may
modify the duty cycle of control valve 86 to achieve the desired flow
rate.

[0044]Controller 114 may also include logic for controlling a maximum
standby pressure. The maximum standby pressure represents the maximum
pressure that can be applied to a hydraulic load. Digital high pressure
standby control generally serves the same purpose as a high standby
relief valve employed in an analog hydraulic system. A pressure relief
valve may, however, be used in conjunction with a digital high pressure
standby control as a backup measure. The maximum standby pressure setting
is typically set lower than the pressure setting of a pressure relief
valve, if one is used. This prevents the pressure relief valve from
opening under normal operating conditions, which may result in an
undesirable loss of energy. Once the pressure reaches the maximum
allowable level, controller 114 may adjust the pulse width of the control
signal used to control operation of the control valve associated with the
hydraulic load to zero. Doing so closes the control valve to prevent any
further increase in pressure.

[0045]Controller 114 may also include logic for controlling a low standby
pressure. Low standby pressure control operates to help insure that a
predetermined minimum pressure is always delivered to a hydraulic load
when the load does not require any flow. Maintaining a minimum standby
pressure may enable the hydraulic load to react in a predictable and
reasonably responsive manner. The low standby pressure can be maintained
by controller 114 generating a pulse width modulated control signal
having narrow pulse width for controlling the control valve associated
with the hydraulic load. The narrow pulse width control signal causes the
valve to have an effective opening that is large enough to allow
sufficient flow to pass through the control valve to compensate for
system leakage while maintaining pressure at the minimum standby pressure
level.

[0046]Low pressure standby control may be used, for example, in
conjunction with a power steering system employing hydraulic cylinder 26.
The low standby pressure typically occurs when the power steering system
is positioned in the neutral position. With the power steering system in
the neutral position, controller 114 may issue a low standby pressure
command signal for instructing hydraulic cylinder control valve 54 to
deliver the requested pressure to hydraulic cylinder 26. The low standby
pressure is sufficient to allow the hydraulic cylinder 26 to firmly
maintain the desired steering geometry of the vehicle and to enable quick
actuation of the steering mechanism. In practice, controller 114 may
formulate the pulse width modulated control signal for operating the
control valve based on a maximum of the requested pressure level and the
low standby pressure level, whichever is higher.

[0047]With continued reference to FIG. 2, control valve 40 is shown to
employ an exemplary forty percent (40%) duty cycle; control valve 70
shown to employ an exemplary thirty percent (30%) duty cycle; control
valve 86 shown to employ an exemplary twenty percent (20%) duty cycle;
and control valve 100 shown to employ an exemplary ten-percent (10%) duty
cycle. It shall be understood that the duty cycles depicted in FIG. 2 are
for illustrative purposes only. In practice, the duty cycle for a given
control valve may differ from that which is shown, and indeed, may vary
with time to accommodate changing load requirements.

[0048]With continued reference to FIGS. 1 and 2, control valves 40, 70,
86, and 100 employ a common operating period, which for purpose of
illustration, may be set at twenty (20) milliseconds. As noted
previously, the actual operating period may vary depending on the
configuration and operational requirements of hydraulic system 10. The
control valves are actuated sequentially one after another in such a
manner that when one valve is closed, or in some instance, nearly closed,
the next valve is opened. Generally, only one valve is fully open at any
given time, although there may be a relatively short period of time
during which the opening and closing sequences of sequentially actuated
valves intersect one another. Each valve is generally opened and closed
only once during a given operating cycle. A single operating cycle
comprises cycling through at least a subset of the available control
valves only once. The sequence in which the valves are cycled may change
between operating cycles.

[0049]When operating hydraulic system 10 there may be instances in which
the flow requirements of the hydraulic loads exceeds the flow output of
pump 12. When that occurs a determination may be made as to what
proportions the available flow will be distributed between the hydraulic
loads. This may be accomplished by assigning each hydraulic load a
priority level. For example, a priority level one (1) may be considered
the highest priority, a priority level two (2) the second highest
priority, and so forth. Each hydraulic load may be assigned a priority
level. The bypass circuit is typically assigned the lowest priority
level.

[0050]Various criteria may be used to determine the priority assignments,
including but not limited to safety concerns, efficiency considerations,
operator convenience, among others. Each hydraulic load may be assigned a
separate priority level or multiple hydraulic loads may be assigned the
same priority level depending on the requirements of the particular
application. The priority level assignment for each load may be saved in
controller 114 such as by way of memory 153, or in the memory or other
tangible storage mechanism of a system level electronic control unit
(ECU) in operational communication with controller 114.

[0051]The available flow may be distributed to the hydraulic loads based
on their priority level ranking, with the hydraulic loads assigned the
highest priority level (i.e., priority level 1) receiving all of the flow
they require, and the remaining hydraulic loads receiving either a
reduced flow or no flow at all. An example of possible priority level
assignments for fluid circuits 32, 34, 36 and 101, and a resulting flow
distribution based on the priority level assignments is shown in Table 1
below. For purposes of this example, it is assumed that hydraulic pump 12
has a maximum output of one-hundred fifty (150) liters/min. For
illustrative purposes, first fluid circuit 32, which includes hydraulic
cylinder 26, is assigned a priority level one. Second and third fluid
circuits 34 and 36 are assigned a priority level two. Bypass fluid
circuit 101, which is typically assigned the lowest priority level, is
assigned priority level three. In this example, the first fluid circuit
requires two-thirds (66.7 percent) of the total available flow, or 100
liters/min. The second and third fluid circuits both require one-third
(33.3 percent) of the available flow, or 50 liters/min. Since the total
flow requirement of all three fluid circuits exceed the available flow
from pump 12, the second and third fluid circuits, which are assigned a
lower priority than the first fluid circuit, will receive only a portion
of their required flow. The first fluid circuit will receive its total
flow requirement of 100 liters/min. This leaves 50 liters/min. to be
distributed between the second and third fluid circuits. Since the second
and third fluid circuits have the same priority level, the remaining 50
liters/min. is divided evenly between the two fluid circuits, with each
circuit receiving 25 liters/min. The bypass fluid circuit receives no
fluid in this example since all of the available fluid is distributed
between the other three fluid circuits. [0052]Total flow rate
available=150 liters/min.

[0053]The order in which the control valves are actuated may have an
effect on the efficiency of the hydraulic system. The valves may be
actuated in sequential order based on various selected criteria, for
example, in order of decreasing or ascending pressure. The order in which
the control valves are actuated may be determined based on the pressure
requirements of the hydraulic loads, for example, hydraulic loads 26, 28,
and 30. Typically, the control valve supplying the hydraulic load with
the highest pressure requirement is actuated first, followed by the
control valve supplying the hydraulic load with the next highest pressure
requirement and so forth down the line until all of the control valves
have been actuated. If a particular hydraulic load does not require
pressure, the control valve associated with the non-operational hydraulic
load will not be opened during that particular operating cycle. Bypass
control valve 100 is typically actuated last, if at all, after all of the
remaining control valves (i.e., control valves 40, 70, and 86) have been
actuated. Once all the control valves have been actuated the present
operating cycle is completed and the next operating cycle may be
commenced.

[0054]An example of a possible sequencing order for control valves 40, 70,
86, and 100 is illustrated graphically in FIG. 5. An upper curve 152 in
the graph represents an exemplary system pressure profile, for example,
as measured by pressure sensor 126 (see FIG. 1). Exemplary individual
channel pressure curves 154, 156 and 158, represent a pressure occurring
at the inlet to hydraulic loads 26, the respective hydraulic load. The
"channel #1 pressure" curve 154 depicts the time varying pressure as
measured at the inlet to hydraulic cylinder 26. The "channel #2 pressure"
curve 156 depicts the time varying pressure as measured at the inlet to
hydraulic motor 28. The "channel #3 pressure" curve 158 depicts the time
varying pressure as measured at the inlet to miscellaneous hydraulic load
30. The generally square-wave curve 160 shown at the bottom of the figure
graphically depicts an opening and closing sequence of control valves 40,
70, 86 and 100. The pulse labeled "#1" depicts an exemplary opening and
closing of control valve 40. The pulse labeled "#2" depicts an exemplary
opening and closing of control valve 70. The pulse labeled "#3" depicts
an exemplary opening and closing of control valve 86. The pulse labeled
"bypass" depicts an exemplary opening and closing of bypass control valve
100. Since hydraulic cylinder 26 has the highest pressure requirement in
this example, control valve 40 will be actuated first, followed in order,
by control valve 70 that controls the operation of hydraulic motor 28,
and control valve 86 that controls the operation of miscellaneous
hydraulic load 30. Bypass control valve 100 is actuated last. The same
sequence may be repeated for subsequent operating cycles provide there is
no change in the pressure requirements of the hydraulic loads that may
require changing the sequencing order.

[0055]The order in which the control valves are sequenced may not always
be consistent. The sequencing order may be varied between operating
cycles, and in some cases midway through an operating cycle, to
accommodate changes in operating conditions, such as load pressure
requirements. If the pressure requirement of a hydraulic load becomes
higher than the pressure requirement of one or more of the remaining
hydraulic loads, the sequencing order may be reordered so that the
control valves continue to be sequenced from the highest pressure
requirement to the lowest pressure requirement. For example, in FIG. 5,
hydraulic cylinder 26 is depicted as having the highest pressure
requirement, followed in order by hydraulic motor 28 and miscellaneous
hydraulic load 30. The control valves are accordingly sequenced in
descending order, with control valve 40 being actuated first, followed in
order by control valves 70 and 86. Bypass valve 100 is actuated last. If
the pressure requirement of miscellaneous hydraulic load 30 were to
become higher than the pressure requirement of hydraulic motor 28, for
example, as shown in FIG. 6A, the sequencing order may be rearrange, such
that control valve 86 is actuated before control valve 70. The revised
sequencing order is illustrated in FIG. 6B. The sequencing order may be
re-evaluated and adjusted if necessary at the beginning of each
subsequent operating cycle. The operating period may also be varied
between operating cycles.

[0056]Improvements in overall system performance may be realized by
adjusting the pulse width of a control valve midway through an operating
cycle to accommodate changes in the flow requirements of the hydraulic
load. This is in contrast to determining the pulse width for each
hydraulic load at the start of an operating cycle and maintaining the
same pulse width for the duration of the operating cycle. Progressive
pulse width control, in which the pulse width is adjusted midway through
the operating cycle, may improve system bandwidth, which is directly
influenced by the system's operating cycle frequency. An exemplary
implementation of progressive pulse width control is illustrated
graphically in FIGS. 8A and 8B. FIG. 8A illustrates an operating cycle in
which the pulse width for each hydraulic load and the bypass (designated
"1", "2", "3" and "bypass" in FIG. 8A) is determined at the beginning of
the operating cycle. In the example illustrated in FIG. 8A, the operating
cycle has progressed to the time identified by the line marked "Current"
in FIG. 8A. Control valve 2 (labelled "2" in FIG. 8A) is currently in the
process of supplying flow to the corresponding hydraulic load. Assume
that midway through its duty cycle there is in increase in the flow
requirement of the hydraulic load associated with control valve 2. To
accommodate the increased flow demand, the pulse width of the control
signal used for controlling control valve 2 may be increased and the
pulse width of the signal for controlling control valve 3 or the bypass
valve may be reduced in proportion to the increase in the pulse width
associated with control valve 2. The changes to the duty cycle to
accommodate the increased flow requirements of the hydraulic load
associated with control valve 2 are reflected in FIG. 8B. Since the flow
requirements of the hydraulic load associated with control valve 1 have
already been satisfied within the current operating cycle, any changes in
its flow requirements will not be accommodated until the next operating
cycle.

[0057]Referring again to FIG. 5, the timing during which one control valve
is closed and the next control valve is opened may affect the efficiency
of the hydraulic system. Effective control of the time delay between
closing one valve and opening the next may help minimize energy losses
that may occur while transitioning between fluid circuits, such as first
fluid circuit 32, second fluid circuit 34, third fluid circuit 36, and
bypass fluid circuit 101 (see FIG. 1). The time delay is identified as
"Δt" in FIG. 5. The first time delay (Δt1) represents
the delay between commencing closing bypass valve 100 and commencing
opening control valve 40. The second time delay (Δt2)
represents the delay between commencing closing control valve 40 and
commencing opening control valve 70. The third time delay
(Δt3) represents the delay between commencing closing control
valve 70 and commencing opening control valve 86. The forth time delay
(Δt4) represents the delay between commencing closing control
valve 86 and commencing opening bypass valve 100.

[0058]Factors that may be considered in determining an appropriate time
delay may include the volume and the compliance of the fluid supply
circuit between pump 12 and control valves 40, 70, 86 and 100. The time
delay is also a function of the pressure difference between fluid
circuits.

[0059]If the time delay between commencing closing one control valve and
commencing opening the next successive control valve is too long, energy
may be wasted as the fluid present in the supply circuit leading to the
control valve is compressed, thereby causing a spike in system pressure.
This phenomenon is depicted graphically in FIG. 7B. The upper graph in
FIG. 7B depicts an exemplary change in system pressure (P) (for example,
the pressure sensed by pressure sensor 126 in FIG. 1) as a first control
valve closes and the next control valve opens. The lower graph in FIG. 7B
graphically depicts an exemplary opening and closing two control valves.
The valves are fully open at (Δor). The left portion of the
lower curve graphically depicts the closing of a first valve and the
right portion of the curve graphically depicts the opening of a second
valve. Because the time delay is short, fluid present in the fluid supply
circuit between the hydraulic pump and the control valve (i.e., pump
discharge passage 22 in FIG. 1) is compressed causing a spike in pressure
that can be observed in the upper pressure curve of FIG. 7B.

[0060]If the delay between commencing closing one valve and commencing
opening the next successive valve is too short, fluid may flow backward
from the previous hydraulic load (valve 1) to the next hydraulic load
(valve 2). This phenomenon is depicted graphically in FIG. 7A. The upper
curve in FIG. 7A depicts an exemplary change in system pressure (P) as a
first control valve closes and the next control valve opens. The lower
curve in FIG. 7A graphically represents an exemplary opening and closing
of the control valves. The valves are fully open at (Δor). In
this example, a second control valve begins to open before a first
control valve has fully closed. Note that the system pressure depicted in
the upper graph of FIG. 7A begins to drop as the first control valve
begins to close. Although having a short time delay may not necessarily
result in a drop in efficiency, unless for example the fluid backflows
from a hydraulic load to a tank, such as fluid reservoir 18 (see FIG. 1),
it nevertheless may be accounted for when determining a control signal
pulse width that will provide the net flow required by the hydraulic
load. Accordingly, it may also be desirable to optimize the time delay
between commencing closing the bypass control valve and commencing
opening the first control valve in the sequence and the time delay
between commencing closing the last control valve in the sequence and
commencing opening the bypass valve. Determining a proper time delay may
entail a compromise between minimizing the amount of backflow occurring
between the control valves, as depicted in FIG. 7A, and minimizing the
occurrence of system pressure spikes, as depicted in FIG. 7B.

[0061]The time delay (Δt) may be determined using the following
equation:

Δt=α*ΔP+TimeDelayAdder

Where:

[0062]Δt (Time Delay) is the time period between commencing to
close one control valve and commencing to open the next subsequent valve
(see for example FIG. 5); [0063]α is a parameter that may be
dependent on various parameters, for example, valve transition speed,
valve friction, pump flow rate, thermal effects, effective bulk modulus
of the hydraulic fluid, and the internal volume of the an internal pump
or the valve manifold; [0064]ΔP is the pressure difference between
the hydraulic load and the outlet of the pump; and

[0065]TimeDelayAdder is an empirically determined correction factor for
optimizing the time delay.

[0066]By way of example, in instances where α is dependent on
manifold volume, pump flow rate, and effective bulk modulus of the
hydraulic fluid, the time delay (Δt) may be determined using the
following equation:

Δ t = Δ PV β Q +
TimeDelayAdder ##EQU00001##

Where:

[0067]Δt (Time Delay) is the time period between commencing to
close one control valve and commencing to open the next subsequent valve
(see for example FIG. 5); [0068]ΔP is the pressure difference
between the hydraulic load and the outlet of the pump; [0069]V is the
fluid volume of the fluid circuit between the pump outlet and the inlet
of the control valve; [0070]β is the effective bulk modulus of the
hydraulic system; [0071]Q is the flow rate of the pump; and
[0072]TimeDelayAdder is an empirically determined correction factor for
optimizing the time delay.

[0073]The bulk modulus may be determined using the following equation:

β = V ∂ P ∂ V = V P
t / V t ##EQU00002##

The bulk modulus varies non-linearly with pressure. The bulk modulus of
the hydraulic fluid is a function of temperature, entrained air, fluid
composition and other physical parameters. The bulk modulus of the
hydraulic system is representative of the volume and rigidity of the
hydraulic system hardware and is a factor in determining an appropriate
time delay. The effective bulk modulus of a hydraulic system is a
compilation of the bulk modulus of the fluid and the bulk modulus of the
system hardware. In practice, the bulk modulus may vary significantly,
and if possible, may be measured to obtain an accurate bulk modulus for
use in computing the time delay. Measurement of the effective bulk
modulus may be accomplished, for example, by monitoring a pressure rise
in hydraulic system 10 as a function of fluid flow from pump 12 with all
the control valves 40, 70, 86 and 100, closed. The pump flow may be
approximated using the following equation:

Pressure rise may be monitored using a pressure sensor (i.e., pressure
sensor 126 in FIG. 1) located in the fluid supply circuit between pump 12
and control valves 40, 70, 86 and 100. A lookup table containing a map of
the effective bulk modulus as a function of pressure may be generated and
stored in memory 163 of controller 114 for use in computing the time
delay.

[0074]The bulk modulus can be mapped during an initial start-up of the
hydraulic system to provide an initial operating map. The bulk modulus
can be measured periodically as the hydraulic fluid heats up until a
steady state condition is reached. Bulk modulus maps for similar system
conditions obtained during previous operating cycles may be compared and
used to evaluate the status of the hydraulic system. For example, a
substantial decrease in bulk modulus may indicate a significant increase
in entrained air in the hydraulic fluid, or an impending failure in a
hydraulic system hose or pipe.

[0075]The TimeDelayAdder parameter included in the equation for computing
the time delay (Δt) is a correction factor for optimizing the time
delay (Δt). The α parameter and the TimeDelayAdder parameter
may be determined empirically. The α term of the time delay
equation, which may correspond, for example, to the equation (ΔP
V/βQ), or another functional relationship, provides an estimate of
the amount of delay between commencing to close one control valve and
commencing to open the next successive valve. Since it is only an
estimate, however, the computed time delay (Δt) may not produce an
optimum balance between minimizing system pressure spikes and backflow
occurring between successively actuated control valves.

[0076]The effectiveness of the time delay (Δt) estimate may be
assessed by computing a corresponding Time Delay Pressure Error that at
least partially accounts for the losses associated with both spikes in
system pressure and backflow from one control valve to the next. The Time
Delay Pressure Error may be computed using the following equation:

[0077]Ppump is a pressure output from pump 12, as detected, for
example, using pressure sensor 126; [0078]Pload is a pressure
delivered to the hydraulic load (i.e., hydraulic loads 26, 28 and 30);
and [0079]ΔPvalve is a steady state pressure drop across the
control valve (i.e., control valves 40, 70, 86 and 100).

[0080]The steady state pressure drop across the control valve
(ΔPvalve) may be obtained from a look-up table stored in
memory 153 of controller 114, wherein the steady state pressure drop is
correlated to the flow rate of pump 12. The flow rate of pump 12 may be
computed using a measured pump RPM, which may be detected, for example,
using speed sensor 124, and the previously described equation for
determining Pump Flow.

[0081]The substance of the Time Delay Pressure Error may be better
understood with reference to FIGS. 9-11. FIG. 9 graphically depicts an
exemplary fluctuation in pressure drop occurring across three separate
control valves (i.e., control valves 40, 70 and 86) as the valves are
successively opened and closed. The three control valves may be actuated
in sequence in the manner previously described. In this example, control
valve 40 is opened first, followed in order by control valve 70 and
control valve 86. The pressure drop across each control valve is tracked
starting from the point when the control valve first begins to open
through to when the valve is fully closed. The steady state pressure drop
across the valves is the same for all three valves and is represented by
the horizontal line denoted as such in FIGS. 9 and 11. It shall be
appreciated, however, that it is not necessary that each valve have the
same pressure drop. Note that the pressure drop curves for successive
control valves may at least partially overlap during the transition
period during which one valve is closing and the next valve is opening.
This is due to the fact that the subsequently actuated valve begins to
open before the previous valve is fully closed.

[0082]As can be observed from FIG. 9, the pressure drop across a given
control valve may vary significantly from the valve's corresponding
steady state pressure drop as the valve transitions between its open and
closed positions. From the pressure drop curves it may be possible to
detect inefficiencies that may be occurring during the transition period.
For example, a spike in the pressure drop across a given control valve in
excess of the steady state pressure drop that occurs as the valve is
opening (i.e., pressure spike 162, 164 and 166 in FIG. 9) may suggest
that the time delay (Δt) is too short, causing fluid to backflow
from the control valve that is closing to the control valve that is
opening. A negative pressure drop across a given control valve that
occurs as the control valve is closing (i.e., negative pressure drop 168,
170 and 172) may indicate that fluid is flowing from the control valve
that is closing to the passage supplying the fluid to the control valve
(e.g., pump discharge passage 22). A spike in the pressure drop across a
given control valve in excess of the steady state pressure that occurs as
the control valve is closing (i.e., pressure spike 167 in FIG. 11) may
indicate that the time delay (Δt) is too long, causing a spike in
system pressure.

[0083]FIG. 11 is an enlarged view of a portion of FIG. 9, illustrating an
exemplary transition period between control valve 70 closing and control
valve 86 opening. Note that there is a spike in the pressure drop across
control valve 40 above the steady state pressure drop that occurs as the
control valve begins to close. This is a due to control valve 40 starting
to close before control valve 70 has started to open. The fluid present
in the fluid supply circuit between hydraulic pump 12 and control valve
40 is compressed as the control valve closes, thereby causing the spike
in system pressure.

[0084]Continuing to refer to FIG. 11, the pressure drop across control
valve 40 begins to drop below the steady state pressure drop as control
valve 70 begins to open, and continues to drop as valve 40 is closed. The
pressure drop across control valve 40 eventually goes negative as valve
40 continues to close and valve 70 continues to open. The negative
pressure drop may indicate the presence of backflow from control valve 40
to pump discharge 22. The spike in pressure drop across control valve 70
may also signal that fluid is back flowing from control valve 40 to
control valve 70. The spike in system pressure and backflow of fluid from
control valve 40 to control valve 70 may have a detrimental affect on
system efficiency. Minimizing these losses may improve the overall
efficiency of the hydraulic system.

[0085]With continued reference to FIG. 11, the Time Delay Pressure Error
at a given point in time, for example time "T" in FIG. 11, may be
computed by summing the amount by which the pressure drop across the
control valve exceeds the steady state pressure drop (identified as
pressure drop "A" in FIGS. 9 and 11) and the amount by which the pressure
drop falls below zero (identified as pressure drop "B" in FIGS. 9 and
11). The first term in the Time Delay Pressure Error
(MAX[(Ppump-(Pload-ΔPvalve),0)]) corresponds to
pressure drop "A" and the second term (ΔBS(MIN
[Ppump-Pload,0])) corresponds to pressure drop "B". A Time
Delay Pressure Error may be computed at various time intervals throughout
the operating cycle. A graph of Time Delay Pressure Errors computed using
the pressure drops from FIG. 9 is shown in FIG. 10. Note that the Time
Delay Pressure Error is zero once the pressure drop across the control
valve reaches steady state.

[0086]The time delay (Δt) may be optimized by minimizing the Time
Delay Pressure Error. This may be accomplished by incrementally varying
the TimeDelayAdder parameter in the time delay (Δt) equation until
a minimum Time Delay Pressure Error is achieved. A new time delay
(Δt) is computed for each TimeDelayAdder value. The corresponding
control valve is then operated using the modified time delay (Δt)
and the resulting pressure drop across the control valve is tracked. A
new Time Delay Pressure Error is computed based on the latest pressure
drop data and compared with the previously computed Time Delay Pressure
Error. This process continues until a minimum Time Delay Pressure Error
is determined. An optimum TimeDelayAdder corresponding to the minimum
Time Delay Pressure Error, along with the corresponding pressure and flow
rate, may be stored in memory 153 of controller 114 in the form of a
lookup table for future reference.

[0087]With reference to FIGS. 1 thru 4, operation of an exemplary
operating cycle of hydraulic system 10 will be described. Exemplary duty
cycles for control valves 40, 70, 86 and 100 are illustrated in FIG. 2.
The time varying fluid output of control valves 40, 70, 86 and 100 is
expressed as a percentage of fluid output of pump 12. The exemplary
operating cycle commences at time equals zero. For purposes of
discussion, it is presumed that hydraulic load 26 initially has the
highest pressure requirement, followed in order by hydraulic load 28 and
hydraulic load 30. The control valves are actuated in descending order,
starting with control valve 40, which controls the hydraulic load having
the highest pressure requirement, followed in order by control valves 70,
86, and 100. The exemplary operating cycle has a duration of twenty (20)
milliseconds, which corresponds to the operating period of each of the
described duty cycles. Two consecutive operating cycles are depicted in
FIGS. 2-4, with the second operating cycle commencing at time equals to
20 milliseconds and ending at time equals forty (40) milliseconds. The
operating cycles for control valve 40, 70, 86 and 100 all start and end
at the same time.

[0088]FIG. 4 graphically describes the time varying relative fluctuations
in fluid pressure occurring down stream of pump discharge port 24, as
detected by pressure sensor 126. The pressure detected by pressure sensor
126 reasonably approximates the pressure occurring at the inlet of the
respective loads when the corresponding control valve is open due to the
relatively low pressure losses that occur within the hydraulic system.

[0089]FIG. 3 graphically describes the time varying relative flow rates
and pressure levels occurring near an inlet of the respective hydraulic
load. In the case of bypass fluid circuit 101, which does not include a
hydraulic load, the pressure and flow rates occur within bypass discharge
passage 108. Due to the relatively low pressure losses that occur within
the system, the pressure occurring near the inlet of the hydraulic load
closely approximates the pressure detected at pump discharge port 24 by
pressure sensor 126. Hence, the inlet pressure curve for a given
hydraulic load, as shown in FIG. 3, generally corresponds to the pressure
occurring at pump discharge port 24 (as shown in FIG. 4) during the
period in which the control valve is open.

[0090]Continuing to refer to FIGS. 1-4, the exemplary operating cycle may
be initiated (at time equals zero in FIGS. 2-4) by controller 114 sending
a control signal to actuator 42 instructing the actuator to open control
valve 40 and establish a fluid connection between inlet port 46 and
discharge port 50. Based on a forty percent (40%) duty cycle, control
valve 40 will remain open for a period of approximately eight (8)
milliseconds. With control valve 40 in the open position, the entire
quantity of fluid discharged from pump 12 will pass through control valve
40 (see FIG. 2) to fluid junction 71. Depending on the flow and pressure
requirements of hydraulic load 26, a portion of the fluid arriving at
fluid junction 71 will be delivered to hydraulic load 26 through
discharge passage 52 and either first supply passage 62 or second supply
passage 64 depending on the current flow setting of hydraulic cylinder
control valve 54. The time varying rate at which fluid is delivered to
hydraulic load 26 is depicted graphically in FIG. 3. The remaining fluid
arriving at fluid junction 71 will pass through supply/discharge passage
73 to accumulator 68 to charge the accumulator. As shown in FIG. 4,
during the period in which control valve 40 is open, the pressure
detected by pressure sensor 126 (which approximates the pressure level
occurring near the inlet port of hydraulic load 26, as shown in FIG. 3)
will begin to rise as a result of hydraulic load 26 restricting the flow
of fluid from pump 12. After control valve 40 has been open for a period
of approximately eight (8) milliseconds, controller 114 may send a
control signal to actuator 42 instructing the actuator to close control
valve 40. With control valve 40 in the closed position, the pressure and
flow rate at fluid junction 71 begins to drop. This in turn causes
pressurized fluid stored in accumulator 68 to be released into discharge
passage 52. As can be observed from FIG. 3, the fluid discharged from
accumulator 68 at least partially compensates for the drop in flow and
pressure occurring within discharge passage 52 due to control valve 40
being closed. The result is a gradual decrease in the fluid flow and
pressure level within discharge passage 52 occurring over a time period
of approximately eight (8) milliseconds to approximately twenty (20)
milliseconds, rather than an abrupt drop that would likely occur if
accumulator 68 were not utilized. The pressure and flow will continue to
drop until control valve 40 is opened during a subsequent operating
cycle, which occurs at time equaling approximately twenty (20)
milliseconds (see FIGS. 2 and 3). The pressure and flow curves will be
substantially the same for subsequent operating cycles so long as there
is no change in the operating conditions.

[0091]Upon closing control valve 40, controller 114 may send a control
signal to actuator 77 instructing the actuator to open control valve 70
and establish a fluid connection between inlet port 72 and discharge port
78. Based on a thirty percent (30%) duty cycle, control valve 70 will
remain open for a period of approximately six (6) milliseconds, starting
at approximately eight (8) milliseconds and ending at approximately
fourteen (14) milliseconds. With control valve 70 in the open position,
the entire flow of fluid discharged from pump 12 will pass through
control valve 70 (see FIG. 2) to fluid junction 85.

[0092]As shown in FIG. 4, the pressure within pump discharge passage 22
(as detected by pressure sensor 126) will initially drop to the level
indicated at a point 174 of the pressure curve upon opening control valve
70. Depending on the flow and pressure requirements of hydraulic load 28,
a portion of the fluid arriving at fluid junction 85 will be delivered to
hydraulic load 28 through hydraulic motor supply passage 80. The time
varying fluid flow rate near an inlet port of hydraulic load 28 is
graphically depicted in FIG. 3. The remaining fluid arriving at fluid
junction 85 will pass through supply/discharge passage 87 to accumulator
84 to charge the accumulator. During the period in which control valve 70
is open (time period between approximately eight (8) milliseconds and
fourteen (14) milliseconds), the pressure detected by pressure sensor 126
(see FIG. 4) and the pressure level near the inlet port of hydraulic load
28 (see FIG. 3) will begin to rise above the initial pressure that
occurred when control valve 70 was first opened (point 174 of FIG. 4).
After control valve 70 has been open for a period of approximately six
(6) milliseconds, controller 114 can send a control signal to actuator 77
causing control valve 70 to close the fluid path between inlet port 72
and discharge port 78. With control valve 70 closed the pressure level
and rate of fluid flow at fluid junction 85 will begin to drop. This will
cause pressurized fluid stored in accumulator 84 to discharge into
hydraulic motor supply passage 80 during the period in which control
valve 70 is closed (time period of 14 milliseconds-28 milliseconds). As
can be observed from FIG. 3, the fluid discharged from accumulator 84 at
least partially compensates for the drop in flow and pressure that occurs
when control valve 70 is closed. The result is a gradual decrease in the
flow rate and pressure level within discharge passage 80 that occurs over
the time period from approximately fourteen (14) milliseconds to
approximately twenty-eight (28) milliseconds. The pressure and flow will
continue to drop until control valve 70 is again opened during a
subsequent operating cycle, which occurs at time equals approximately
twenty-eight (28) milliseconds. The pressure and flow curves will be
substantially the same for subsequent operating cycles so long as there
is no change in the subsequent operating conditions.

[0093]Upon closing control valve 70, controller 114 may send a control
signal to actuator 93 instructing the actuator to open control valve 86
to establish a fluid connection between inlet port 88 and discharge port
96. Based on a twenty percent (20%) duty cycle, control valve 86 will
remain open for a period of approximately four (4) milliseconds, starting
at approximately fourteen (14) milliseconds and ending at approximately
eighteen (18) milliseconds. With control valve 86 in the open position,
the entire flow of fluid discharged from pump 12 will pass through
control valve 86 (see FIG. 2) to fluid junction 97. As shown in FIG. 4,
the pressure within pump discharge passage 22 (as detected by pressure
sensor 126) will initially drop to the level indicated at point 176 of
the pressure curve upon opening control valve 86. Depending on the flow
and pressure requirements of hydraulic load 30, a portion of the fluid
arriving at fluid junction 97 will be delivered to hydraulic load 30
through hydraulic load supply passage 94. The time varying fluid flow
rate near an inlet port of hydraulic load 30 is graphically depicted in
FIG. 3. The remaining fluid arriving at fluid junction 97 will pass
through supply/discharge passage 99 to accumulator 95 to charge the
accumulator. During the period in which control valve 86 is open (time
period of approximately fourteen (14) milliseconds to approximately
eighteen (18) milliseconds), the pressure detected by pressure sensor 126
(see FIG. 4) and the pressure occurring near the inlet port of hydraulic
load 30 (see FIG. 3) will begin to rise above the initial pressure that
occurred when control valve 86 was first opened (point 176 of FIG. 4).
After control valve 86 has been opened for a period of approximately four
(4) milliseconds, controller 114 may send a control signal to actuator 93
causing control valve 86 to close the fluid path between inlet port 88
and discharge port 96. With control valve 86 in the closed position, the
pressure level and rate of fluid flow at fluid junction 97 will begin to
drop. This will cause pressurized fluid stored in accumulator 95 to be
discharged into hydraulic load supply passage 94 during the period in
which control valve 86 is closed (time period approximately eighteen (18)
milliseconds to approximately thirty-four (34) milliseconds). As can be
observed from FIG. 3, the fluid discharged from accumulator 95 at least
partially compensates for the drop in flow and pressure that occurs when
control valve 86 is closed. The result is a gradual decrease in the flow
rate and pressure level within discharge passage 94 that occurs over the
time period between 18 milliseconds and 34 milliseconds. The pressure and
flow will continue to drop until control valve 86 is again opened during
a subsequent operating cycle (at time equals approximately thirty-four
(34) milliseconds). The pressure and flow curves will be substantially
the same for subsequent operating cycles so long as there is no change in
the subsequent operating conditions.

[0094]Upon closing control valve 86, control valve 100 may be selectively
opened to dump any excess pressure present within pump discharge passage
22 to fluid reservoir 18. Controller 114 may send a control signal to
actuator 112 instructing the actuator to open bypass control valve 100 to
establish a fluid connection between inlet port 102 and discharge port
110. Based on a ten percent (10%) duty cycle, control valve 86 will
remain open for a period of two (2) milliseconds, starting at eighteen
(18) milliseconds and ending at twenty (20) milliseconds. The closing of
control valve 86 at approximately twenty (20) milliseconds corresponds to
the end of the current operating cycle and the beginning of the
subsequent operating cycle. With control valve 100 in the open position,
the entire flow of fluid discharged from pump 12 will pass through
control valve 100 (see FIG. 2) and bypass discharge passage 108 to
reservoir return passage 66. As shown in FIG. 4, the pressure within pump
discharge passage 22 (as detected by pressure sensor 126) will drop to
the level indicated at point 178 of the pressure curve when control valve
100 is opened, and will remain at that pressure until control valve 100
is closed at time equals approximately twenty (20) milliseconds. After
bypass control valve 100 has been open for a period of two (2)
milliseconds, controller 114 may send a control signal to actuator 112
causing control valve 100 to close the fluid path between inlet port 102
and discharge port 110.

[0095]The current exemplary operating sequence is completed when bypass
control valve 100 is closed. A subsequent operating sequence may be
commenced by actuating control valve 40 and repeating the previously
described operating sequence. If there a change in operating conditions,
for example, wherein a pressure requirement of a hydraulic load has
increased or decreased, the affected control valve duty cycle may be
reevaluated and adjusted as necessary to accommodate the changed
operating condition.

[0096]With regard to the processes, systems, methods, etc. described
herein, it should be understood that, although the steps of such
processes, etc. have been described as occurring according to a certain
ordered sequence, such processes could be practiced with the described
steps performed in an order other than the order described herein. It
further should be understood that certain steps could be performed
simultaneously, that other steps could be added, or that certain steps
described herein could be omitted. In other words, the descriptions of
processes herein are provided for the purpose of illustrating certain
embodiments, and should in no way be construed so as to limit the claimed
invention.

[0097]It is to be understood that the above description is intended to be
illustrative and not restrictive. Many embodiments and applications other
than the examples provided would be apparent to those of skill in the art
upon reading the above description. The scope of the invention should be
determined, not with reference to the above description, but should
instead be determined with reference to the appended claims, along with
the full scope of equivalents to which such claims are entitled. It is
anticipated and intended that future developments will occur in the arts
discussed herein, and that the disclosed systems and methods will be
incorporated into such future embodiments. In sum, it should be
understood that the invention is capable of modification and variation
and is limited only by the following claims.

[0098]All terms used in the claims are intended to be given their broadest
reasonable constructions and their ordinary meanings as understood by
those skilled in the art unless an explicit indication to the contrary in
made herein. In particular, use of the singular articles such as "a,"
"the," "said," etc. should be read to recite one or more of the indicated
elements unless a claim recites an explicit limitation to the contrary.